In recent years, electronic equipment, such as cellular phones and notebook computers, use devices whose resistance has a positive temperature coefficient, or PTC devices, as protective devices for secondary cells. Demand exists for such electronic equipment to implement high functionality, long-hour operability, and higher efficiency. Under the circumstances, secondary cells are required to implement large capacity and high voltage. In association with these requirements, PTC devices are required to withstand high voltage. At present, PTC devices of about 8 V are in practical use. For withstanding higher voltage, insulation performance in a current limiting condition, which is an OFF state, must be enhanced; i.e., dielectric strength must be enhanced. Mainstream materials for matrices of conventional PTC devices are solid materials, such as ceramics and polymers. For example, polyethylene-based PTC devices and barium-titanate-based PTC devices are used (refer to Patent Documents 1 and 2).
FIG. 7 is a pair of views showing the principle of a basic operation of a conventional PTC device, wherein (a) shows an ON state, and (b) shows an OFF state. The PTC device has a structure in which conductive particles serving as filler are mixed in a solid insulator, such as ceramics or a polymer; i.e., in a solid matrix. Normally, the PTC device is in an ON state, in which the conductive particles are in contact with one another and bridge the electrodes as shown in (a) of FIG. 7, thereby forming a conductive path. When the PTC device is brought into a high-temperature state as a result of inflow of overcurrent thereto, the conductive path is cut as a result of evaporation of the conductive particles or expansion of the solid matrix as shown in (b) of FIG. 7. As a result, resistance increases abruptly, and the PTC device is brought into a cutoff/current-limiting state; i.e., an OFF state. In this manner, in the conventional PTC device configured such that the conductive particles are present in the solid matrix, an OFF state is established by cutting the path of conductive filler through expansion of the matrix.
At present, PTC devices of low dielectric strength are widely used as protective devices for lithium ion cells for use in cellular phones and computers. However, in association with implementation of large-capacity cells, PTC devices of high dielectric strength are required. For a structural reason, a solid matrix involves the generation of cracks and voids in principle when the solid matrix expands. Since gas is present in such cracks and voids surrounded by the solid matrix having high dielectric constant, an electric field concentrates in cracks and voids, so that discharge is apt to be generated in cracks and voids. For this reason, a PTC device using a solid matrix suffers material deterioration caused by gaseous discharge, resulting in impairment in recovering characteristics. Thus, under present circumstances, difficulty is encountered in fabricating a reliably usable PTC device of 8 V or higher, depending on a PTC device structure.
Under the above-mentioned technological circumstances, the inventors of the present invention filed an application for a self-recovery current limiting fuse using a liquid matrix, which can suppress the generation of cracks and voids as compared with a solid matrix (refer to Patent Document 3). The self-recovery current limiting fuse using a liquid matrix disclosed in Patent Document 3 enhances dielectric strength through suppression of generation of cracks and voids and implements self-restoration characteristics by means of dielectrophoretic force of solid conductive particles generated through application of voltage. Thus, by means of solid conductive particles being mixed in a liquid matrix; i.e., solid conductive particles being fluidly dispersed in a liquid matrix, contact electric-resistance, or ON resistance, can be lowered; through enhancement of dielectric strength, a secondary cell having high rated voltage is protected; the range of applications is expanded; efficiency is improved; charging time is shortened; and maintenance-free operation is attained.
According to Patent Document 3, fusion cutting of a fuse element by overcurrent is utilized for operational change from an ON state to an OFF state. Specifically, when overcurrent flows between electrodes in an ON state, in which solid conductive particles are chained in a liquid matrix for establishment of a conducting state, Joule heat is generated in the liquid matrix. As a result, the solid conductive particles evaporate and disperse, whereby a cutoff/current-liming operation is effected, thereby establishing a cutoff/current-limiting state. Because of utilization of evaporation of solid conductive particles, particularly in the case of use of a fuse element having high melting point, some difficulty is involved in transfer to an OFF state. Also, the self-recovery current limiting fuse of Patent Document 3 does not have an emergency trip function.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H6-215903    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2005-285999    Patent Document 3: Japanese Patent No. 3955956